Chicago 2-flats, 3-flats legality and zoning

[Daniel Dem]

Been searching for this for a while and I can't find a clear answer from the city or from other investors.

What is the checklist of items that I would need to satisfy to get a 1920s Ukranian 2-flat and/or a 1960s 3-flat to be able to legally rent out 2 units plus a basement unit?  Most of the ones I've seen are in RS-2 and RS-3 zones, which I understand are zoned for single family residences with the ability to add only one additional dwelling unit if grandfathered in before 1957.  One would then assume that all buildings in RS-2 and RS-3 zones built after 1957 with 3 units are illegal buildings, correct?  Yet I see them advertised by agents everywhere.  They claim "we have the papers" yet, I haven't seen any.  What are these papers that one would need?  A certificate of occupancy explicitly stating it is a 3-unit?  A zoning amendment?

Thanks.